Hi Steve, I have had two trikes electrified, and both were with Heinzmann steel geared motors. The first used an electrified mtb wheel, 7spd freewheel, the second, an ICE flat folding trike, had the motor fitted in the 20" wheel, with 9spd freewheel. Both conversions were carried out by myself with the wheels supplied by Steve Punchard of
http://www.electricmountainbikes.com/. He also gave help and supplies throughout. Steve also has info re which current hubs will take cassette/freewheel, disc compatibility etc.
I can give you quite a bit of info re performance, fitting and suitability of the different motors etc as well as pics of the trikes. Also thoughts on dishing/strength reliability re trikes, and just remembered STABILITY!
Kind regards
Nigel